There are a number of known devices and methods for production of energy from ocean currents.
Norwegian patent application no. 1999 1984 describes a plant for production of electrical energy from ocean and river currents. The plant is in its entirety located below the surface of the water and comprises several turbines with blades, a support system, a guy system and a generator. The turbines' shafts are oriented normal to the water's direction of motion and the blades are wing-shaped so that the turbine rotates in the same direction, irrespective of the water's direction of motion. The turbine shafts are supported in a framework with buoyancy tanks, mounted on the support and bearing system, and the plant is composed of modules. The plant has positive buoyancy regulated by the buoyancy tanks and a guy system secured below the surface of the water, with the result that the plant is held under the surface by the guy system. The plant employs traditional blades.
U.S. Pat. No. 4,864,152 describes a floating water current power plant consisting of a ring pontoon which by means of a bollard is moored to anchors on the bottom. All the turbines can be replaced and are mounted on a common beam and can be pivoted up to the surface as one unit within the area defined by the ring pontoon. The power plant can move around the bollard, whose top end is connected to a fore pontoon and whose bottom end is secured to the anchors.
U.S. Pat. No. 5,440,176 describes a submersible water turbine plant comprising turbines/generators in different combinations suspended beneath a submerged platform of the tension leg type.
Another example of a floating tidal power plant is mentioned in the applicant's own international patent application PCT/NO02/00249 (International publication no. WO03/006825 A1). This plant comprises a platform with a number of buoyancy elements, preferably one in each corner of the platform, thereby satisfying the requirement for stability both in the lateral and the longitudinal direction, with the minimum of displacement. A number of supporting arms extend from the platform to each side, each holding a generator housing. The supporting arms are hinged at the attachment point in the platform.
The known plants are substantially designed with a view to being placed in coastal waters, where wind and wave forces are more moderate than they can be on the open sea. However, there exists a substantial energy recovery potential in placing power plants at sea or in other demanding environments. There is therefore a need for a plant for production of energy from currents in water which can withstand greater environmental forces than hitherto known plants. Norwegian patent application no. 20070228 describes a plant for production of energy from currents in water, comprising a structure for submerging in the water and a plurality of turbine modules connected to the structure. The plant comprises a plurality of columns which are connected to the structure at one of their respective ends and furthermore slidably connected to a buoyancy element, and each column can be selectively and releasably attached to the buoyancy element.
Plants located at sea, however, are exposed to substantial dynamic loads induced by waves and wind. In the above-mentioned plant for location at sea, the part of the buoyancy element located above the surface of the water may be subjected to wave and wind loads which can cause the whole plant to be set in relatively substantial motion, particularly about the plant's transversal axis. This represents structural challenges as well as having an unfavourable effect on the turbines' efficiency. There is therefore a need for a plant which is less sensitive to substantial environmental forces.